Gemini South Reveals First Nitrogen-Sequence Wolf-Rayet Nucleus of a Planetary Nebula

March 14, 2012

Figure 1. Gemini South GMOS spectrum of the WN central star of IC4663
(black) with model atmosphere (red). The model has a very hot
stellar temperature of 140,000 degrees. According to the standard
classification scheme for massive WN stars, the subtype is WN3 due to
the absence of carbon and neutral helium emission lines.

Figure 2. The planetary nebula IC4663 viewed by the HST (inset; red,
green and blue channels made from ionized hydrogen and nitrogen, doubly ionized oxygen and visual light, respectively) and
GMOS at Gemini South (background; doubly ionized oxygen). The central
star is at the center of the image which measures one arcminute on each
side. The full size inner portion may be found separately here.

Astronomers using the Gemini South telescope have discovered the first
clear-cut example of a windy helium and nitrogen rich central star of
a planetary nebula. This work provides the best evidence yet that there is another
way to make hydrogen poor stars in the late-life stages of low mass stars -
like the Sun.

The international team, led by Brent Miszalski of the South African
Astronomical Observatory and the Southern African Large Telescope,
were searching for binary companions to central stars of planetary
nebulae, when they stumbled across the rarest of stellar gems.
Gemini observations of the nucleus of IC4663 (Figure 1) revealed a
peculiar mix of helium and nitrogen emission lines, unique and
entirely out of place for a planetary nebula, but nonetheless an
apparent clone of high mass Wolf-Rayet stars.

Planetary nebulae nuclei are the extremely hot, inert cores of low
mass stars like the Sun, which are not far away from retiring as
Earth-sized white dwarfs. The atmospheres of most nuclei show
absorption lines of hydrogen and helium similar to white dwarfs. Many
other strange flavors also exist. Around 100 planetary nebulae with
Wolf-Rayet type nuclei are known, uniquely displaying emission lines
of carbon, oxygen and helium, as a result of their powerful winds.
These are the low-mass cousins of massive carbon-rich (WC-type)
Wolf-Rayet stars, the final pre-supernova phase of very massive stars.
A second flavor of massive Wolf-Rayet star, rich in helium and
nitrogen (WN-type), are also common, but no clear-cut counterparts
amongst planetary nebulae nuclei have been identified to date. While a
handful of candidates have been identified, most cannot be
unequivocally distinguished apart from ejecta known to surround some
massive WN stars. A well known example is the WN star WR124, and its
nebula M1-67, which is a proven massive star.

Is IC4663 just another case of a massive star with a confused
identity? Team member Professor Paul Crowther, from the University of
Sheffield, explains, "IC4663 may walk and talk like a duck, but our
analysis reveals a completely different beast." It would have likely
taken billions of years for IC4663 to reach old age, whereas it’s
heftier cousins could get there in just a few million years. According
to Dr Miszalski, the properties of the central star were "just right"
for the team to prove that it is the first low-mass counterpart to
nitrogen-rich WN stars. Relative to its massive cousins, it is
exceptionally faint, and possesses an elliptical inner nebula,
exquisitely captured by the Hubble Space Telescope (Figure 2), that is
characteristic of other planetary nebulae. In addition, Gemini images
reveal the presence of a faint halo (also seen in Figure 2). Haloes are widely
accepted as a telltale signature of a previous cool giant phase, one
that massive Wolf-Rayet stars do not experience.

Dr Miszalski’s team discovered that IC4663 hosted a helium and
nitrogren rich central star that unambiguously had a WN-type spectrum,
a composition that had never previously been predicted by theoretical
models that aim to trace the evolutionary steps of low mass stars like
the Sun. Models are able to reproduce the composition of carbon-rich
planetary nebulae nuclei, but not IC4663. According to Dr Miszalski, "if our understanding of Solar-type stars were complete, then the
central star of IC4663 simply should not exist!".

It is hard enough to explain why Wolf-Rayet central stars lack
hydrogen, but it is even harder to come to terms with the extremely
helium-rich nature of IC4663. Its existence is the first solid
evidence that there's a second way to make hydrogen poor central
stars, producing a helium-rich atmosphere instead of the more common
carbon-rich atmosphere. Further work is needed to identify the
evolutionary origins of the helium-rich composition. Although IC4663
does not appear to be a binary at the present time, binary stellar
evolution may be the answer, since it may be the product of a stellar
merger. Binary central stars are the leading explanation for the
perplexing variety of exquisite shapes of planetary nebulae and
several new discoveries have been made by Dr Miszalski and colleagues
in recent years such that we now know of around 50 such systems. This paper has been accepted for publication in Monthly Notices of Royal Astronomical Society. The pre-print is available at http://arxiv.org/abs/1203.3303.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Maunakea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Científica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.